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XXL Note relative, to the supposed Origin of the Deficient Rays in the Solar Spec- 
trum ; being an Account of an Experiment made at Edinburgh during the Annular 
Eclipse of IbthMay 1836. By James D. Forbes, Esq. F.R.SS. L. 8$ E. F.G.S. 8$c., 
and Professor of Natural Philosophy in the University of Edinburgh. 
Received May 25, — Read June 2, 1836. 
The occurrence of the late solar eclipse, which was annular at Edinburgh, sug- 
gested and offered a simple method of conducting the following inquiry. 
The deficiency of rays of light of certain definite degrees of refrangibility in the 
solar spectrum, was discovered by Dr. Wollaston*, but excited little attention 
until its rediscovery by M. Fraunhofer'!". Since that period it has been a frequent 
subject of inquiry whence that deficiency proceeds. We first are tempted to sus- 
pect that the deficient rays may have been lost in passing through the prism used. 
But as in most cases the dark lines are the same, whatever he the material of the 
prism, and whatever the length of the path described by the light within it, this sup- 
position is not tenable^. 
If these lines were all or principally owing either to the absorptive action of any 
matter which may exist in the planetary spaces, or to the effect of the earth’s own 
atmosphere, we should have the same lines exhibited in the spectra of the fixed stars 
as in that of the sun. Sir David Brewster indeed states § that he has been able to 
discover certain lines of the spectrum which are due to the action of the earth’s 
atmosphere alone, since they are seen at small angular elevations of the sun above 
the horizon, and disappear when its altitude is greater. But these are not very nu- 
merous or important compared to the great mass of lines. Sir David Brewster 
having also observed that the lines produced by the absorption of nitrous acid gas 
upon artificial light are in many respects similar to those existing in the solar rays, 
is naturally led to attach considerable probability to the idea that the solar light is 
originally complete, and that the deficient rays have been stopped in passing through 
the sun’s own atmosphere ||. This atmosphere might be supposed to contain nitrous 
acid, or some similar gas, as a constituent. 
* Philosophical Transactions, 1802. 
t Memoirs of the Bavarian Academy; Schumacher’s Astronomische Abliandlungen, 1823; Gilbert’s 
Annalen, 1823. 
J Rudberg, Biblioth&que Universelle, Janvier 1836. 
§ Edinburgh Transactions, xii. 528. 
|| I do not know with whom the idea of the absorptive action of the sun’s atmosphere originated. The 
